Compact cladding-pumped planar waveguide amplifier and fabrication method

ABSTRACT

A low-cost, high performance cladding-pumped planar waveguide amplifier and fabrication method, for deployment in metro and access networks. The waveguide amplifier has a compact monolithic slab architecture preferably formed by first sandwich bonding an erbium-doped core glass slab between two cladding glass slabs to form a multi-layer planar construction, and then slicing the construction into multiple unit constructions. Using lithographic techniques, a silver stripe is deposited and formed at a top or bottom surface of each unit construction and over a cross section of the bonds. By heating the unit construction in an oven and applying an electric field, the silver stripe is then ion diffused to increase the refractive indices of the core and cladding regions, with the diffusion region of the core forming a single mode waveguide, and the silver diffusion cladding region forming a second larger waveguide amenable to cladding pumping with broad area diodes.

[0001] The present invention relates to and claims priority under 35 USC120 to Provisional Application No. 60/276,812 filed Mar. 15, 2001,entitled “Erbium Doped Waveguide Amplifier and Method Thereof”.

[0002] The United States Government has rights in this inventionpursuant to Contract No. W-7405-ENG-46 between the United StatesDepartment of Energy and the University of California for the operationof Lawrence Livermore National Laboratory.

FIELD OF THE INVENTION

[0003] The present invention relates to optical amplifiers incommunications networks. The present invention relates more particularlyto a low-cost, compact, high performance rare earth-doped planarwaveguide amplifier for use in metropolitan area networks, and a methodfor fabricating the same.

BACKGROUND OF THE INVENTION

[0004] Over a decade ago, optical amplifiers were developed to replaceelectronic repeaters in the telecommunications network due to the highcost of building and operating electronic repeaters, as well as theintroduction of bottlenecks caused by electronic regeneration. Opticalamplifiers enable “optical transparency,” allowing the data stream totravel thousands of miles without encountering the need for electricalregeneration. In particular, long-haul transmission designers started toembrace communications at a wavelength of 1.55 μm using erbium (Er)doped silica glass fiber amplifiers (EDFAs), leading to the massivebuild-up of the present day communications network infrastructure. Thesubstantial gain bandwidth of the EDFA, together with the use ofwavelength division multiplexing (WDM) to combine and separate signals,has enabled transmission of 160 individual wavelengths in the fiber,with the bit rate for each wavelength channel usually 2.5 Gb/s, andcontinuing to increase (to 10 and even 40 Gb/s).

[0005] Since the introduction of optical amplifiers, thetelecommunications infrastructure and market have grown explosively dueto the burgeoning demand for communications and information. Inparticular, the increased demand has led to the growth in infrastructureof “metropolitan area networks” (MANs or simply metro) where broadbandwidth is brought closer to and made directly accessible byconsumers. FIG. 1 illustrates the core relationship of MANs 11 withaccess networks, such as broadband cable TV. While the direct opticalamplification with EDFAs (13 in FIG. 1) has been successful in meetingthe need for raw bandwidth in the long-haul portion (12 in FIG. 1) ofthe network, it is considered too expensive for widespread use in MANs,and very bulky with fiber lengths of about 40 m.

[0006] Thus, in order to address the increased need for raw bandwidthfor metro and consumer access to broadband, metro networks require acompact, reliable, high-performance 1.55 μm optical amplifier capable oflow cost commercial mass production. Such an optical amplifier wouldenable new functionality (i.e. cost, integration, and/or alternativehosts) for the metro and access networks. It is expected that thetransmission rates per wavelength will continue to increase from 2.5Gb/s to 40 Gb/s. Without low cost amplifiers widely implementedthroughout the metro and access networks, the higher transmission rateswill reduce the reach of the transmitters, limiting the ability toupgrade the system.

SUMMARY OF THE INVENTION

[0007] One aspect of the present invention includes a method offabricating a monolithic cladding-pumped optical waveguide amplifier,comprising the steps of: fabricating a multi-layer planar constructionhaving n rare earth-doped core layer(s) and n+1 cladding layers inalternating layered arrangement, and top and bottom surfaces revealing across-section of the layers, wherein the core layer(s) has a higherrefractive index than the cladding layers; and diffusing ions through atleast one of the top and bottom surfaces into the core layer(s) and thecladding layers to form ion-diffused regions thereof having respectivelyincreased refractive indices, the ion-diffused regions forming anion-diffused block laterally situated between non-diffused regions of apair of partially-diffused cladding layers. In this manner, theion-diffused region(s) of the core layer(s) forms a signal waveguide(s)for carrying signals therethrough, and the ion-diffused regions of thecladding slabs together form a cladding-pump waveguide for opticallypumping the signals carried through the signal waveguide(s).

[0008] Another aspect of the present invention includes a method offabricating a monolithic cladding-pumped optical waveguide amplifier,comprising the steps of: bonding n core slab(s) with n+1 cladding slabsin alternating layered arrangement to produce a multi-layer planarconstruction, wherein the core slab(s) has a refractive index greaterthan the cladding slabs; slicing the multi-layer planar constructioninto at least two multi-layer planar units each having top and bottomsurfaces revealing a cross-section of the slab bonding; on eachmulti-layer planar unit, producing by metal deposition andphotolithography an ion-diffusable metallic stripe on one of the top andbottom surfaces and over the core slab(s) and the cladding slabs; anddiffusing ions from the metallic stripe into the core slab(s) and thecladding slabs to form ion-diffused regions of the core slab(s) and thecladding slabs having respectively increased refractive indices, theion-diffused regions forming an ion-diffused block laterally situatedbetween non-diffused regions of a pair of partially-diffused claddingslabs. In this manner, the ion-diffused region of the core slab forms asignal waveguide for carrying signals therethrough, and the ion-diffusedregions of the cladding slabs together form a cladding-pump waveguidefor optically pumping the signals carried through the signal waveguide.

[0009] Another aspect of the present invention is a method offabricating a monolithic cladding-pumped optical waveguide amplifierfrom a multi-layer planar substrate, said multi-layer planar substratehaving n rare earth-doped core layer(s) and n+1 cladding layers inalternating layer arrangement, and top and bottom surfaces revealing across-section of the layers, wherein the core layer(s) has a higherrefractive index than the cladding layers, said method comprising thesteps of: diffusing ions through at least one of the top and bottomsurfaces into the core layer(s) and the cladding layers to formion-diffused regions thereof having respectively increased refractiveindices, the ion-diffused regions forming an ion-diffused blocklaterally situated between non-diffused regions of a pair ofpartially-diffused cladding layers, wherein the ion-diffused region(s)of the core layer(s) forms a signal waveguide(s) for carrying signalstherethrough, and the ion-diffused regions of the cladding slabstogether form a cladding-pump waveguide for optically pumping thesignals carried through the signal waveguide(s).

[0010] Another aspect of the present invention includes a monolithiccladding-pumped optical waveguide amplifier fabricated according to oneof the methods described above.

[0011] Some of the advantages of the monolithic cladding-pumped opticalwaveguide amplifier and method of the present inventions include thelowering of manufacturing costs due to the incorporation of standardlithographic techniques that naturally extend to commercial massproduction. The compact slab architecture also enables the use of lowcost high-power broad stripe diode pumps which can further lower costs.The waveguide amplifier is suitable for use in both single and multimodehigh-power waveguide amplifiers. Moreover, the compact cladding-pumpedslab architecture of the waveguide amplifier offers increasedfunctionality and performance to low and high-power waveguide lasersinvolving different hosts and laser ions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings, which are incorporated into and form apart of the disclosure, are as follows:

[0013]FIG. 1 is an organizational illustration of a metro core andaccess networks for which the present invention is ideally suited.

[0014]FIG. 2 is a front perspective view of a first cladding slab and acore slab prior to bonding.

[0015]FIG. 3 is a front perspective view following FIG. 2 of the firstcladding slab and a core slab after bonding.

[0016]FIG. 4 is a front perspective view following FIG. 3 afterreduction of the core slab.

[0017]FIG. 5 is a front perspective view following FIG. 4 after bondinga second cladding slab to the core slab to produce a planar slabwaveguide construction.

[0018]FIG. 6 is a front perspective view following FIG. 5 of the firstcladding illustrating the configurations of the planar unit waveguidesformed from slicing.

[0019]FIG. 7A is a front perspective view of a planar unit waveguideafter deposition of a silver stripe and aluminum layer on the top andbottom surfaces, respectively, via photolithography.

[0020]FIG. 7B is a front view of the planar unit waveguide of FIG. 7Aillustrating the ion diffusion step.

[0021]FIG. 8 is a front perspective view of the planar unit waveguidefollowing ion diffusion of FIG. 7B and removal of the aluminum layer andexcess silver stripe.

[0022]FIG. 9 is a front perspective view of the planar unit waveguidecoupled to a diode pump in an illustrative arrangement.

[0023]FIG. 10 is a top view of the planar unit waveguide of FIG. 9illustrating signal and diode light paths for optic pumping.

[0024]FIG. 11 is a top view of a first embodiment of a planar unitwaveguide arranged in a first amplifier arrangement for optic pumping.

[0025]FIG. 12 is a top view of a second embodiment of a planar unitwaveguide arranged in a second amplifier arrangement for optic pumping.

[0026]FIG. 13 is a top view of a third embodiment of a planar unitwaveguide arranged in a third amplifier arrangement for optic pumping.

[0027]FIG. 14 is a top view of a fourth embodiment of a planar unitwaveguide arranged in a fourth amplifier arrangement for optic pumping.

[0028]FIG. 15 is a top view of a fifth embodiment of a planar unitwaveguide illustrating a first arrangement and use of the ion-diffusedregions of the outer cladding slabs as an integrated optic coupler.

[0029]FIG. 16 is a top view of a sixth embodiment of a planar unitwaveguide illustrating a second arrangement and use of the ion-diffusedregions of the outer cladding slabs as an integrated optic coupler.

[0030]FIG. 17 is a top view of a seventh embodiment of a planar unitwaveguide illustrating another arrangement and use of an ion-diffusedregion of one of the outer cladding slabs as an integrated opticalcoupler.

[0031]FIG. 18 is a top view of an eighth embodiment of a planar unitwaveguide illustrating another arrangement and use of the ion-diffusedregions of the outer cladding slabs as integrated optical couplers onopposite ends of the waveguide.

[0032]FIG. 19 is a schematic view of a diffusion process using a saltbath with enhancement by an electric field.

DETAILED DESCRIPTION

[0033] Turning now to the drawings, FIGS. 2-8 show a first preferredfabrication method of the present invention for producing a compactcladding-pumped planar waveguide amplifier. The first preferredembodiment of the present invention combines slab bonding and metal iondiffusion to create a cladding-pumped planar waveguide amplifier,commonly referred to as an EDWA (erbium-doped waveguide amplifier). Theprocess of the present invention utilizes standard lithographictechniques that reduce costs by naturally extending to commercial massproduction. Moreover, the multi-layer planar construction producedaccording to the process of the present invention is configured tooptimize the use of inexpensive broad stripe diode pumps to furtherincrease cost savings.

[0034]FIG. 2 shows a first cladding slab 100 and a core slab 101 bothhaving a generally rectangular shape. The first cladding material 100(and the second cladding material 104 in FIG. 5) is an undoped glass,while the core slab 101 is a rare earth-doped glass. It is notable thatwhile erbium is a commonly used dopant in communications applications(ergo the common acronym “EDWA”) the present invention is not limitedonly to such. The architecture of the cladding-pumped planar waveguideamplifier could be readily applied to other gain media such as Yb:glass,Nd:glass, Tm:glass, or other rare earth-doped glass with no othervariation than simply using a different rare earth as the dopant in anion exchangeable glass. The glass composition is unimportant other thanits ability to undergo ion exchange; any ion exchangeable glass willsuffice. Additionally, co-doped media may alternatively be utilized,such as Yb: Er, to reduce the absorption length. In any case, the dopinglevel is sufficient for efficient absorption and high gain. And therefractive index of the core slab 101 is greater than the refractiveindex of the first cladding slab 100 (as well as the refractive index ofthe second cladding slab 104) so as to enable waveguiding. It is notablethat the respective indices of the first and second cladding slabs 100and 104 need not be equal, only that they be less than the refractiveindex of the core slab 101.

[0035]FIG. 3 shows the first cladding slab 100 and the core slab 101bonded together at a first bond 103. Various bonding techniques may beutilized to effect slab bonding. Silica glue based on potassiumhydroxide and dissolved silicon dioxide is one preferred method ofoptically bonding the slabs according to a publication entitled, “AnUltra-Precision and Reliable Bonding Technique Using Hydroxide-CatalyzedSurface Hydration/Dehydration,” the disclosure of which is incorporatedherein by reference. A combination of pressure and temperature near thediffusion point (approximately 400° C.) cures the glue and anneals outdefects. Alternatively, phosphate glue (similar to silica glue) may beutilized in a phosphate glue bonding process provided by Schott GlassTechnologies of Duryea, Pa. Still other alternative methods includeusing an optical epoxy polymer or thermal diffusion bonding to createthe optical bond. In the case of thermal diffusion bonding, a bond isformed by optically contacting the surfaces with temperature andpressure. Moreover, the thermal diffusion bond is annealed at atemperature near the thermal diffusion temperature. In any case, thecore slab 101 and the cladding slabs 100, 104 are preferably firstpolished to about λ/20 wave flatness prior to the bonding step tominimize loss at the bonds.

[0036] Next, the exposed surface 102 (in FIG. 3) of the core slab 101 isthen reduced preferably to approximately 10 microns, a thicknesssuitable for single mode operation for a given Δn=0.004, V =2.242. FIG.4 shows the exposed surface 102′ of the core slab 101 after reduction.Various reduction methods may be utilized, such as grinding/polishing.An alternative to polishing would be to acid etch the surface to therequired thickness. In any case, FIG. 5 shows the next step of bonding asecond cladding slab 104 to the exposed surface of the reduced core slab101 at a second bond 105 using one of the bonding techniques describedwith respect to the first cladding slab 104. This produces a multi-layerplanar construction 106 having low loss bonds. As shown in FIG. 5, themulti-layer planar construction 106 generally has miniature-scaledimensions, (except the core slab which is micro-scale) measurable incentimeters. Exemplary dimensions are shown in FIG. 5 having a height of10cm, a length of 10 cm and a width of 1 cm. Moreover, it is appreciatedthat additional core slabs and cladding slabs may be bonded inalternating slab (or layer) arrangement, with each core slab sandwichedbetween a pair of cladding slabs. The ratio of rare earth-doped corelayers or slabs to cladding layers or slabs is n:n+1 where n is aninteger ≧1.

[0037] It is notable here that in lieu of bonding the first claddingslab 100 with the core slab 101, a functionally equivalent step-indexedstructure may be obtained by sputtering the doped material onto aportion of the first cladding slab 100, to create a doped core region ofthe first cladding slab 100. In this manner, it is possible to eliminatethe first bonding/gluing step discussed above, as well as the need toreduce the surface via grinding/polishing. Additionally, the entiremulti-layer planar construction 106 could alternatively be formed usingfiber-pulling techniques. The fiber pulling technique allows easyinitial manufacture of the desired device at a much larger scale calleda preform. This large scale version can then be pulled to the desiredscale and dimensions while preserving the geometric shape and relativescale of preform components. Pulling is accomplished in a draw towerwhich is a large tower with the preform at the top and a furnaceimmediately below which heats the end of the preform close to melting.The end of the preform is then stretched by a combination of gravity andtension to form a fiber of the desired dimensions. In this case, thefinal product is at a mesoscale between preform and fiber, which isoften called “cane”. The technique at this scale is called “pullingcane”.

[0038] In FIG. 6, the multi-layer planar construction 106 is shownsliced perpendicular to the large faces of the structure 106 and intoindividual multi-layer planar units 110 that will each becomesingle-mode channel waveguides. Each of the sliced units 110 is relativethin (shown as 0.5 cm) in height compared to the width and lengththereof. As can be seen, the direction of the slicing produces a topsurface 111 and a bottom surface 112 on each unit 110, each revealing across-section of the slab bonding, i.e. the sandwiched arrangement ofthe core slab 101 between the cladding slabs 100 and 104.

[0039] In FIG. 7A-B, one of the multi-layer planar units 110 is shownwith a metallic stripe 200 formed using photolithography. Generally, themetallic stripe 200 has a metal composition capable of ion diffusinginto the multi-layer planar unit 110 to raise the refractive index.Preferably, silver (Ag) is known to have suitable properties for raisingthe refractive index upon diffusion and is used herein as an exemplarymetal of the present invention. Thus, formation of the silver stripe 200begins by depositing a layer of silver, such as by vapor deposition, atone of the top or bottom surfaces of the multi-layer planar unit 110 tocoat over the exposed core slab 101 and target portions 300, 301 in FIG.7B. A photoresist is then applied and UV light is used to expose theresist through a photomask having a stripe width of approximately 120microns. It is notable that the stripe width will always cover theentirety of the exposed core slab 101 (or core slabs if arranged in amulti-channel array as shown in FIGS. 14-18). Additionally, the stripewidth will cover the respective target portions of each cladding slab,with each target portion comprising all or a portion of a cladding slab.For example, the target portions 300 and 301 in FIG. 7B comprise only apart of outer cladding slabs 100 and 104, respectively. In FIG. 14,however, the target portions of respective inner cladding slabs1406-1409 comprise the entirety of each inner cladding slab. Onepossible variation would be to form an angled or tapered shape for thesilver stripe 200 in order to produce an angled or tapered claddingregion which increases diode absorption and pump efficiency (see FIGS.13 and 15-18). The exposed resist is acid etched, removing all thesilver from the multi-layer planar unit except the 120 micron silverstripe 200. Next, a layer of aluminum 201 is vapor deposited over bothtop and bottom faces of the waveguide 110 to serve as electricalcontacts 306, 307 of an electric circuit 305.

[0040] As shown in FIG. 7B, by applying an electric field produced bythe electric circuit 305 and heating the photolithographed planar unitin an oven to approximately 200-350° C., the silver stripe 200 diffusesthrough the top surface and into an ion diffusion region 303 of the coreslab 101 and ion diffusion regions 302 and 304 of the cladding slabs 100and 104, respectively. The application of the electric field and theheating may be separately applied to cause diffusion. It is notable thatthe glass slabs have a material composition capable of ion diffusion ofthe selected metal (e.g. Ag+ ion diffusion). For ion diffusion to occur,a modifier must be present in the glass, such as K+, Na+, or some othermodifier ion which may be displaced by the metal diffusion ion. In FIG.7B, silver ions are shown displacing sodium ions in a downward directionwhile the aluminum 201 remains at the surfaces. The higherpolarizability of the silver ions raises the refractive index by as muchas 0.04. Since this process is controlled by diffusion, the silverconcentration and likewise the refractive index change follows acomplementary error function profile versus distance into the waveguide.By careful control of the silver layer thickness and temperature orelectric field strength, the maximum index change and diffusion depthcan be chosen to restrict the waveguide to a single mode. Typicaldiffusion depth is shown in FIG. 7B to be approximately 4-8 microns.Applicants have observed a suitable silver layer thickness between200-300 nm. The exact dimensions and diffusion depths can be varied toproduce multimode waveguides or accommodate larger apertures, and higherpower diode pump sources. It is notable that the diffusion region couldalso be “buried” by a second ion diffusion process which causes thesilver diffusion region to diffuse further into the glass forming awaveguide below the surface of the glass. To complete the fabricationprocess, an acid solution is then used to remove excess silver and cleanthe surface of the newly formed monolithic cladding-pumped planarwaveguide amplifier 120, shown in FIG. 8.

[0041] As an alternative to metal deposition and diffusion, FIG. 19illustrates the use of a salt bath (e.g. silver nitrate, AgNO₃) toaccomplish ion diffusion. The salt bath technique for ion diffusioncreates ion exchange or diffusion conditions by placing the substrate ina bath of molten salt containing the ions to be exchanged. Then the bathand multi-layer planar unit 110 are placed in an electric fieldpotential which facilitates the movement of ions into the substrate. Itis appreciated that even with no electric field applied, diffusion willtake place on both surfaces equally, but at a slower rate. In order totarget only the central portion of the unit 110 comprising the core slab101 and adjacent portions of the cladding slabs 101, 104, a photoresist500 may be utilized. In this manner, ion-diffusion takes place onlythrough the exposed areas of the photoresist 500 to form ion-diffusedregions 302, 303, and 304 of the first cladding 100, the core slab 101,and the second cladding 104, while protecting respective non-diffusedregions 305 and 306 of cladding slabs 104 and 100.

[0042] In either method of ion diffusion, the refractive index of theion diffusion region 303 of the core slab 101 is increased over therefractive indices of all surrounding regions to form a single-modechannel waveguide, while the ion diffusion regions 302 and 304(hereinafter “cladding regions”) of the cladding slabs 100 and 104 forma second larger cladding pump waveguide amenable to cladding pumpedsystems. The ion-diffused regions together form an ion-diffused blocklaterally situated between non-diffused regions, e.g. 305 and 306, of apair of partially-diffused cladding layers or slabs, e.g. 100 and 104.The non-diffused regions operate to define the outer boundaries of thesecond cladding-pump waveguide.

[0043] As noted above, possible alternatives to diffusion of silver ions(by silver deposition of salt bath) might include any ion-diffusablemetal which raises the refractive index in the substrate upon iondiffusion. This includes, inter alia, alkali metals from further downthe periodic table such as rubidium or cesium, as well as monovalenttransitions metals such as mercury or thallium. There is also thepossibility of diffusing ions with higher charge states such as divalentzinc, but these can be less mobile and can require charge compensationin the medium (For every zinc which diffuses into the medium twomonovalent ions such as sodium or potassium must leave, and the zinc nowoccupies the space where two ions existed previously. Anotherpossibility can be substitution for a divalent ion in the glass such asmagnesium, but again the mobility is less (this makes diffusion moredifficult, and could result in defect formation).

[0044] As shown in FIGS. 9 and 10, the fabricated cladding-pumped planarwaveguide amplifier 120 may be coupled to and utilized with a broad areastripe diode laser 400 which pumps light 401 at 980 nm into the largercladding region 302, 304. The high numerical aperture of the claddingregion 302, 304 allows efficient coupling of the large-area,high-divergence multimode diode emission from a broad area stripe diode.As the 980 nm diode pump light 401 travels down the cladding region 302,304 and crosses the doped core region 303, the diode light 401 isabsorbed to provide amplification to an incoming signal, such as 402.Predicted gains of 20-30 dB are possible in a 5 cm long channelwaveguide. It is notable that the waveguide amplifier shown in FIG. 10illustrates the delimited regions defined subsequent to the iondiffusion step. Five regions are shown: the non ion-diffused region 305of the second cladding slab 104, the ion-diffusion region 304 of thesecond cladding slab 104, the ion-diffusion region 305 of the core slab,the ion-diffusion region 302 of the first cladding slab 100, and thenon-ion-diffused region 306 of the first cladding slab 100, with theregions having respective refractive indices of n₁, n₂, n₃, n₄, and n₅.It is appreciated that in order to effect the cladding pumping of thepresent invention when a signal is guided through the ion-diffusionregion 303, the refractive index n₃ must be greater than all otherindices, and both n₂ and n₄ must each be greater than n₁, or n₅.Expressed as an inequality, n₃>n₂,n₄ >n₁,n₅. It is also appreciatedthat, while preferred, n₂ need not equal n₄, and n₁ need not equal n₅.

[0045] The cladding-pumped planar waveguide amplifier 120 of the presentinvention is suitable for applications such as integrated amplifiers forinternet and telecommunications, and both single and multimodehigh-power waveguide amplifiers. While not being bound by any particulartheory, it is believed that this compact cladding-pumped slabarchitecture offers increased functionality and performance to low andhigh-power waveguide lasers involving different hosts and laser ions.Moreover, lower manufacturing costs are achieved by incorporatingstandard lithographic techniques that naturally extend to commercialmass production. The compact slab architecture also enables the use oflow cost high-power broad stripe diode pumps which can further lowercosts.

[0046] Exemplary waveguide amplifiers are shown in FIGS. 11-14illustrating various configurations of the present invention. FIG. 11shows one embodiment of an optic amplifier 1100 having a single diodepump 1101 arranged at one end of the cladding pumped waveguide amplifier1120, with a signal input and output 1102 entering and exiting at anopposite end. FIG. 12 illustrates an asymmetric cladding arrangement ofanother optic amplifier embodiment 1200 having an enlarged claddingregion 1201 and a smaller cladding region 1202 on the opposite side ofthe core slab 1205. Moreover, two broad area diode pumps 1203 and 1204are provided at opposite ends of the cladding pumped waveguide amplifier1220, with a signal entering the waveguide amplifier 1220 at one endindicated at arrow 1206, and exiting at the opposite end indicated atarrow 1207. FIG. 13 illustrates another asymmetric cladding regionarrangement of an optic amplifier 1300 with one of the cladding regions1301 having an angled shape to effect more efficient absorption asdiscussed previously. In this embodiment, a diode pump 1302 is coupledto a wide end of the angled cladding region 1301, and a signal enters atarrow 1303 and exits at arrow 1304. FIG. 14 illustrates an opticamplifier 1400 having an array of core waveguides 1401-1405 sandwichedbetween internal cladding slabs 1406-1409 and a pair of outer claddingslabs 1410 and 1411 in a monolithic construction formed according to theprocess of the present invention. The core waveguides and the claddingslabs are arranged in alternating slab (or layer) arrangement. The corewaveguides 1401-1405 form separate channels for guiding multiple signalsthrough the planar waveguide amplifier 1420. In this configuration thetwo diode pumps 1414 and 1415 generate light which crosses all of thecore waveguides 1401-1405 to amplify the signals guided there through.

[0047] And in FIGS. 15-18, various planar waveguide amplifierconfigurations are shown with outer cladding regions (i.e. the iondiffusion regions of the target portions of the outer cladding slabsproduced by diffusion of a shaped silver stripe) having angled ortapered configurations. The tapered or otherwise convergingly angledshapes of the outer cladding regions form and operate as an integratedoptic coupler extending from a narrow tapered end out to a broad inputend for optimally coupling to an optic pump and increasing diodeabsorption and pump efficiency. In FIG. 15, outer cladding regions 1501and 1502 have wide ends coupled to diode pumps 1503 and 1504 to amplifysignals entering at an end 1505 and exiting at the opposite end 1506. InFIG. 16, outer cladding regions 1601 and 1602 have wide ends which areangled themselves to change the coupling angle and input from the diodepumps 1603 and 1604. Signals enter at end 1605 and exit at opposite end1606. In FIG. 17, only one outer cladding region 1701 is asymmetricallytapered for coupling to a single diode 1702. Here too, the outercladding region 1701 is tapered toward the core waveguides followingsignal propagation from an input end 1703 to an exit or output end 1704.And in FIG. 18, outer cladding regions 1801-1804 are tapered toward thecore waveguides from opposite ends, with signals entering at end 1805and exiting at the opposite end 1806.

[0048] The foregoing is illustrative of the present invention and is notto be construed as limiting thereof. The invention is defined by thefollowing claims, with equivalents of the claims to be included therein.

I claim:
 1. A method of fabricating a monolithic cladding-pumped opticalwaveguide amplifier, comprising the steps of: fabricating a multi-layerplanar construction having n rare earth-doped core layer(s) and n+1cladding layers in alternating layered arrangement, and top and bottomsurfaces revealing a cross-section of the layers, wherein the corelayer(s) has a higher refractive index than the cladding layers; anddiffusing ions through at least one of the top and bottom surfaces intothe core layer(s) and the cladding layers to form ion-diffused regionsthereof having respectively increased refractive indices, theion-diffused regions forming an ion-diffused block laterally situatedbetween non-diffused regions of a pair of partially-diffused claddinglayers, wherein the ion-diffused region(s) of the core layer(s) forms asignal waveguide(s) for carrying signals therethrough, and theion-diffused regions of the cladding slabs together form a cladding-pumpwaveguide for optically pumping the signals carried through the signalwaveguide(s).
 2. The method as in claim 1, wherein the multi-layerplanar construction is fabricated by bonding n core slab(s) with n+1cladding slabs in alternating layered arrangement.
 3. The method as inclaim 2, wherein the multi-layer planar construction is fabricated atleast in part by bonding a first cladding slab to a first core slab,reducing a thickness of the first core slab, and bonding a secondcladding slab to the reduced first core slab opposite the first claddingslab.
 4. The method as in claim 3, wherein the thickness of the firstcore slab is reduced to about 10 μm.
 5. The method as in claim 3,wherein the thickness of the first core slab is reduced by polishing. 6.The method as in claim 3, wherein the thickness of the first core slabis reduced by chemical etching.
 7. The method as in claim 2, wherein thecore slab(s) and the cladding slabs are surface figured to about λ/20wave flatness prior to slab bonding for minimizing loss at the bonds. 8.The method as in claim 2, wherein a silica glue is used to effect slabbonding.
 9. The method as in claim 2, wherein a phosphate glue is usedto effect slab bonding.
 10. The method as in claim 2, wherein an epoxypolymer is used to effect slab bonding.
 11. The method as in claim 2,wherein the slabs are bonded by diffusion bonding.
 12. The method as inclaim 1, wherein the multi-layer planar construction is fabricated by afiber pulling technique.
 13. The method as in claim 1, furthercomprising producing by metal deposition and photolithography anion-diffusable metallic stripe on one of the top and bottom surfaces andover the core layer(s) and the cladding layers; and wherein the step ofdiffusing ions comprises diffusing metallic ions from the metallicstripe into the core layers(s) and the cladding layers.
 14. The methodas in claim 13, wherein the ion-diffusable metal is silver.
 15. Themethod as in claim 13, wherein the metallic ions are diffused byapplying an electric field between the top and bottom surfaces.
 16. Themethod as in claim 15, wherein the metallic ion diffusion is enhanced byheating the multi-layer planar construction.
 17. The method as in claim16, further comprising controlling at least one parameter of metallicstripe thickness, heating temperature, and electric field strength, toselect maximum refractive index change and diffusion depth.
 18. Themethod as in claim 15, further comprising depositing aluminum over themetallic stripe to provide electrical contact during application of theelectric field, and subsequently removing the aluminum and excessmetallic stripe after diffusion.
 19. The method as in claim 1, whereinthe step of diffusing ions comprises placing the multi-layer planarconstruction in a salt bath.
 20. The method as in claim 19, wherein thestep of diffusing ions further comprises applying an electric fieldbetween the top and bottom surfaces to enhance the diffusion rate anddepth.
 21. The method as in claim 1, further comprising slicing themulti-layer planar construction into at least two multi-layer planarunits each having top and bottom surfaces revealing a cross-section ofthe layers; and wherein the diffusion of ions occurs through at leastone of the top and bottom surfaces of each multi-layer planar unit. 22.The method as in claim 1, wherein the rare earth-doped core layer(s) isdoped with erbium.
 23. A monolithic cladding-pumped optical waveguideamplifier produced according to the method of claim
 1. 24. The opticalwaveguide amplifier as in claim 23, wherein at least one of theion-diffusion regions of the partially-diffused cladding layers isshaped to form an integrated optic coupler extending from a narrowtapered end out to a broad input end for optimally coupling to an opticpump.
 25. A method of fabricating a monolithic cladding-pumped opticalwaveguide amplifier, comprising the steps of: bonding n rare earth-dopedcore slab(s) with n+1 cladding slabs in alternating layered arrangementto produce a multi-layer planar construction, wherein the core slab(s)has a refractive index greater than the cladding slabs; slicing themulti-layer planar construction into at least two multi-layer planarunits each having top and bottom surfaces revealing a cross-section ofthe slab bonding; on each multi-layer planar unit, producing by metaldeposition and photolithography an ion-diffusable metallic stripe on oneof the top and bottom surfaces and over the core slab(s) and thecladding slabs; and diffusing ions from the metallic stripe into thecore slab(s) and the cladding slabs to form ion-diffused regions of thecore slab(s) and the cladding slabs having respectively increasedrefractive indices, the ion-diffused regions forming an ion-diffusedblock laterally situated between non-diffused regions of a pair ofpartially-diffused cladding slabs, wherein the ion-diffused region ofthe core slab forms a signal waveguide for carrying signalstherethrough, and the ion-diffused regions of the cladding slabstogether form a cladding-pump waveguide for optically pumping thesignals carried through the signal waveguide.
 26. The method as in claim25, wherein the multi-layer planar construction is produced at least inpart by bonding a first cladding slab to a first core slab, reducing athickness of the first core slab, and bonding a second cladding slab tothe reduced first core slab opposite the first cladding slab.
 27. Themethod as in claim 26, wherein the thickness of the first core slab isreduced to about 10 μm.
 28. The method as in claim 26, wherein thethickness of the first core slab is reduced by polishing.
 29. The methodas in claim 26, wherein the thickness of the first core slab is reducedby chemical etching.
 30. The method as in claim 25, wherein the coreslab(s) and the cladding slabs are surface figured to about λ/20 waveflatness prior to slab bonding for minimizing loss at the bonds.
 31. Themethod as in claim 25, wherein a silica glue is used to effect slabbonding.
 32. The method as in claim 25, wherein a phosphate glue is usedto effect slab bonding.
 33. The method as in claim 25, wherein an epoxypolymer is used to effect slab bonding.
 34. The method as in claim 25,wherein the slabs are bonded by diffusion bonding.
 35. The method as inclaim 25, wherein the ion-diffusable metal is silver.
 36. The method asin claim 25, wherein the metallic ions are diffused by applying anelectric field between the top and bottom surfaces.
 37. The method as inclaim 36, wherein the metallic ion diffusion is enhanced by heating themulti-layer planar construction.
 38. The method as in claim 37, furthercomprising controlling at least one parameter of metallic stripethickness, heating temperature, and electric field strength, to selectmaximum refractive index change and diffusion depth.
 39. The method asin claim 36, further comprising depositing aluminum over the metallicstripe to provide electrical contact during application of the electricfield, and subsequently removing the aluminum and excess metallic stripeafter diffusion.
 40. The method as in claim 25, wherein the core slab(s)is doped with erbium.
 41. A monolithic cladding-pumped optical waveguideamplifier produced according to the method of claim
 25. 42. The opticalwaveguide amplifier as in claim 41, wherein at least one of theion-diffusion regions of the partially-diffused cladding slabs is shapedto form an integrated optic coupler extending from a narrow tapered endout to a broad input end for optimally coupling to an optic pump.
 43. Amethod of fabricating a monolithic cladding-pumped optic waveguideamplifier from a multi-layer planar substrate, said multi-layer planarsubstrate having n rare earth-doped core layer(s) and n+1 claddinglayers in alternating layer arrangement, and top and bottom surfacesrevealing a cross-section of the layers, wherein the core layer(s) has ahigher refractive index than the cladding layers, said method comprisingthe steps of: diffusing ions through at least one of the top and bottomsurfaces into the core layer(s) and the cladding layers to formion-diffused regions thereof having respectively increased refractiveindices, the ion-diffused regions forming an ion-diffused blocklaterally situated between non-diffused regions of a pair ofpartially-diffused cladding layers, wherein the ion-diffused region(s)of the core layer(s) forms a signal waveguide(s) for carrying signalstherethrough, and the ion-diffused regions of the cladding slabstogether form a cladding-pump waveguide for optically pumping thesignals carried through the signal waveguide(s).
 44. The method as inclaim 43, further comprising producing by metal deposition andphotolithography an ion-diffusable metallic stripe on one of the top andbottom surfaces and over the core layer(s) and the cladding layers; andwherein the step of diffusing ions comprises diffusing metallic ionsfrom the metallic stripe into the core layers(s) and the claddinglayers.
 45. The method as in claim 44, wherein the ion-diffusable metalis silver.
 46. The method as in claim 44, wherein the metallic ions arediffused by applying an electric field between the top and bottomsurfaces.
 47. The method as in claim 46, wherein the metallic iondiffusion is enhanced by heating the multi-layer planar construction.48. The method as in claim 47, further comprising controlling at leastone parameter of metallic stripe thickness, heating temperature, andelectric field strength, to select maximum refractive index change anddiffusion depth.
 49. The method as in claim 46, further comprisingdepositing aluminum over the metallic stripe to provide electricalcontact during application of the electric field, and subsequentlyremoving the aluminum and excess metallic stripe after diffusion. 50.The method as in claim 43, wherein the step of diffusing ions comprisesplacing the multi-layer planar construction in a salt bath.
 51. Themethod as in claim 50, wherein the step of diffusing ions furthercomprises applying an electric field between the top and bottom surfacesto enhance the diffusion rate and depth.
 52. The method as in claim 43,further comprising slicing the multi-layer planar construction into atleast two multi-layer planar units each having top and bottom surfacesrevealing a cross-section of the layers; and wherein the diffusion ofions occurs through at least one of the top and bottom surfaces of eachmulti-layer planar unit.
 53. The method as in claim 43, wherein the rareearth-doped core layer(s) is doped with erbium.
 54. A monolithiccladding-pumped optical waveguide amplifier produced according to themethod of claim
 43. 55. The optical waveguide amplifier as in claim 54,wherein at least one of the ion-diffusion regions of thepartially-diffused cladding layers is shaped to form an integrated opticcoupler extending from a narrow tapered end out to a broad input end foroptimally coupling to an optic pump.